1. Field of the Invention
This invention relates generally to a system and method for estimating the speed of an alternator and, more particularly, to a system and method for estimating the speed of a vehicle alternator by identifying the ripple frequency in a rectified alternator signal, such as a current or voltage, using spectrum analysis and converting the ripple frequency to the alternator speed.
2. Discussion of the Related Art
Vehicles typically include an alternator that provides electrical power to recharge a vehicle battery and operate the vehicle electrical systems when the vehicle is running. The alternator includes a pulley that engages a flexible belt driven by the vehicle engine, where the belt also drives other vehicle components, such as a power steering pump, an air compressor, etc. As the belt rotates the alternator pulley, the pulley rotates a rotor within a stator of the alternator, where the rotor typically has a permanent magnet and the stator includes one or more wound coils. A typical vehicle alternator will include three stator windings that are 90° out-of-phase with each other that generate three AC signals that are sent to a three-phase diode bridge rectifier to convert the AC signals to a DC signal suitable for recharging the battery. The alternator will also include a voltage regulator that regulates the DC signal. The diode bridge rectifier generates an AC ripple signal on the DC signal as a factor of the conversion. Various filtering techniques and other operations can be performed to reduce the ripple current generated by the rectifier conversion, but there still will be some level of AC ripple on the DC signal that can be detected.
It is desirable during vehicle operation to know the rotational speed of the alternator for various reasons, such as determining whether the belt driving the alternator pulley is slipping. If the belt is not slipping, then the alternator speed is the engine speed times the ratio of the alternator pulley. Therefore, by knowing the alternator speed, the pulley ratio and the engine speed, it can be determined whether the belt is slipping. Further, by knowing the alternator speed, damage to the belt or even the alternator itself can be determined if that rotational speed is not smooth.